Method for making boundary-layer flow conditions visible

ABSTRACT

Boundary-layer flow conditions of gases are made visible by applying to the surface of a moving or stationary structural body to be exposed to the flow a reactive layer of at least one chemical color indicator, such as an acid-base indicator and/or a Redox indicator or a liquid-crystal indicator, such as a cholesterinic liquid. Then the body is exposed to a flow of gas, such as air, which gas may contain a reagent. The chemical color indicator can also be applied together with gelling means and a moisture binder. The chemical color indicator can also be absorbed by a high-contrast, absorbent paper which is then applied to the body. A metal or plastic foil coated with a binder and/or indicator can also be used for this purpose. A boundarylayer flow pattern image is produced, which can subsequently be recorded by known means.

United States. Patent [1 1 Urban 1 Jan. 22, 1974 METHOD FOR MAKING2,627,600 2/1953 Rines 346/74 R x BOUND ARY L AYER FLOW CONDITIONS3,357,024 12/1967 Webb 346/1 VISIBLE Inventor: Gerd Urban, Erlangen,Germany Siemens Aktiengesellschaft, Munich, Germany Filed: July 7, 1972Appl. No.: 269,822

Assignee:

Foreign Application Priority Data July 7, 1971 Germany 2133865References Cited UNITED STATES PATENTS 6/1923 Thomson 178/88 PrimaryExaminer-Joseph W. Hartary Attorney, Agent, or Firm-Hugh A. Chapin [5 7]ABSTRACT Boundary-layer flow conditions of gases are made visible byapplying to the surface of a moving or stationary structural body to beexposed to the flow a reactive layer of at least one chemical colorindicator, such as an acid-base indicator and/or a Redox indicator or aliquid-crystal indicator, such as a cholesterinic liquid. Then the bodyis exposed to a flow of gas, such as air, which gas may contain areagent. The chemical color indicator can also be applied together withgelling means and a moisture binder. The chemical color indicator canalso be absorbed by a high-contrast, absorbent paper which is thenapplied to the body. A metal or plastic foil coated with a binder and/orindicator can also be used for this purpose. A boundarylayer flowpattern image is produced, which can subsequently be recorded by knownmeans.

23 Claims, 10 Drawing Figures 1 ill METHOD FOR MAKING BOUNDARY-LAYERFLOW CONDITIONS VISIBLE BACKGROUND OF THE INVENTION 1. Field of theInvention This invention relates to a method for making boundary-layergas flow conditions visible. More particularly, it relates to a methodfor producing an image of the laminar and/or turbulent gas flow patternabout a structural body by color reactions with chemical indicatorsystems.

2. Description of the Prior Art In measuring flow, the known aerodynamicmeasurement methods which furnish data on velocity, direction andpressure require probes of a mechanical, pneumatic, electrical ormagnetic nature with which measuring and indicating instruments areassociated. These known methods have the disadvantage that forrepresenting a three-dimensional flow field, a very large number ofmeasurements must be performed with defined space coordinates. Even ifthe investigation is confined to the field of the boundary layer flow, asubstantial effort is still necessary. For recording flow patterns inrotating equipment, probe methods are technically not suited. The sameis true regarding the optical methods which use interference, schlierenor shadow effects.

I For these reasons, means have been utilized for some time to make flowvisible, which are simpler to use and furnish instantaneous images ofthe flow pattern as well as contour patterns. For gas flow, for example,the filament, dust and flake methods, as well as smoke and liquid-mistmethods, have been used. These methods are described in the book by W.Wuest, Stroemungsmesstechnik, Friedr. .Vieweg, Braunschweig 1969), p.151 to 159. Similar methods are applicable to the flow of liquids.

By means of the flow filament method and the dustprecipitation methods,it is possible to make visible the flow separation and the transitionpoint, i.e., the transition from laminar to turbulent flow. Thedetermination of the transition point is important because turbulentflow creates a substantially higher friction resistance than laminarflow. The filament method shows up the transition point by the flutter.In boundary layers, it can also be recognized by the difference in rapidevaporation of liquids or solids placed there.

These known measurement methods cannot be applied in practice torotating equipment because they are either far too expensive to use, orare technically not suitable for methods such as the probe method.Stroboscopic methods with plastic particles introduced into the airstream, and other known means for making flow visible have also beenfound unsuitable for displaying the flow in vacuum cleaner blowers andfans for cars. Thus, in summary, none of these known methods aresatisfactory.

SUMMARY OF THE INVENTION The present invention provides a method formaking boundary-layer flow conditions of gases visible, by ap plying atleast one chemical color indicator in a thin reactive layer to thesurface of a structural body which is to be exposed to the gas flow andthen subjecting the structural body to the gas flow for the purpose ofproducing a visible boundary-layer flow image. The boundary-layer flowimage may be recorded, after it has been made visible, by photographingthe image in color. In the alternative, the chemical color indicator maybe applied to the structural body by first binding it to a smoothabsorbent paper and then applying the paper to the structural body, incases where the structural body has smooth surfaces and a simple shape.After the structural body has been subjected to the gas flow and afterthe chemical color image of the boundary layer of the boundary-layerflow has been produced on the paper, the paper is stripped off thestructural body; this paper provides a record of the flow.

The chemical color indicator to be applied to the structural body may bean acid-base indicator or a Redox indicator or a liquid-crystalindicator. If an acidbase indicator is used, a buffer may be applied tothe acid-base indicator prior to applying the indicator to thestructural body. The buffer to be mixed with the acid-base indicator maybe an alkaline buffer or an acid buffer.

If a liquid-crystal indicator is used, the liquid-crystal indicator maybe applied by spraying it on a low-boiling solvent, so that the solventevaporates leaving a film on the structural body. The liquid-crystalindicator may be a cholesterinic liquid.

A reagent may be added to the gas flow prior to subjecting thestructural body to the gas flow for the purpose of facilitating thecolor reaction of the reactive layer of the chemical color indicator onthe structural body. The reagent may be added to the gas flow byconducting the gas flow over the gaseous reagent or by bubbling the gasflow through an aqueous solution of the reagent.

The chemical color indicator may also be applied in a moisture-retainingadditive or in combination with a gelling agent and a moisture retainer.

This method may also include the additional step of applying a foil tothe structural body prior to subjecting the body to the gas flow, incases where the body has a plane surface. It may be a metal foil or aplastic foil and may be coated with a binding agent.

DESCRIPTION OF THE DRAWINGS FIG. I is a boundary-layer flow imageproduced by the method disclosed in Example 1.

FIG. 2 is a boundary-layer flow image produced by the method disclosedin Example .2

FIG. 3 is a boundary-layer flow image produced by he method qisclssdinfizsam ls .3-

FIG. 4 is a boundary-layer flow image produced by the method disclosedin Example 8.

FIG. 5 is a boundary-layer flow image produced by the ho d scl sed i mpFIGS. 6 and 7 are boundary-layer flow images produced by the methoddisclosed in Example 11.

FIGS. 8, 9, and 10 are boundary-layer flow images produced by themethods disclosed in Example 14.

DETAILED DESCRIPTION being exposed to the flow. The structural body isthen subjected to the gas flow, particularly to air flow, which may bemixed with a reagent. The boundary-layer flow image thus produced may besubsequently recorded or developed by known methods. I

In the practice of the method according to this invention, differencesin the gas flow affect the adjoining surfaces, and they become visibleas color changes. The chemical color indicator systems, i.e., thereaction layer and the reagent contained inthe flowing gas, should besensitive and respond rapidly. For this reason, ion reactions betweenthe reagent and the reactive layer are advantageous.

With the method of this invention, the intensity and direction of theboundary layer flow can be made visible by feeding the reagent directlyin with the gas flow. The flow corresponding to the instantaneous statecan be made visible dynamically. In this manner one can obtain quicklyan overview of the flow conditions if they are changed by a differentvelocity or mechanical adjustment. Furthermore, the transition pointscan be made visible. The progression of the coloring with the exposuregives information regarding the flow direction and the difference inrapid color deepening gives information regarding the relative flowvelocity. ln turbulent boundary-layers, sharply outlined, intenselycolored spots are produced before the other areas are colored.

By means of chemical or physical-chemical bonding of the pure reagentsrequired for the color change, one obtains excellent adhesion, whichwithstands even high centrifugal forces. For example, sorption onanodized '(eloxadized) aluminum is particularly suitable. Although sucha reactive layer of the chemical color indicator is extremely thin andprovides sharply drawn contours, its color intensity is only barelyadequate, for the same reason. With other materials it was foundadvantageous to bind the indicators by means of moistureretainingadditives such as gelatine or agar-agar, or by highly viscoushygroscopic solvents. The reactive layer should be free of air bubbles,and be 0.05 to 0.5 mm thick. The thinner the layer with the same areadensity, the sharper will be the outlines of the flow pattern image. Onthe other hand, the greater the sensitivity for the chemical reagent,the more the binding agent remains moist, as the material transfer fromthe flowing medium into the reactive layer is accelerated by itssolubility. A combination of gelling and moisture-binding agents, suchas glycerin, glycol and glycol ether, yields smoother, highly adhesivereactive layers, which also absorb the reagent readily and areparticularly suited for rotatingstructural parts.

The flow patterns are delineated most clearly by the direct color changeof the surface areas in contact with the flow. Particularly well suitedare a colorless, transparent background and a white to light, opaquebody. On all dark materials, the color contrast must be sufficientlyenhanced.

The indicator preparations used in accordance with this invention canreadily be washed off from the surface of the test body exposed to theflow and be replaced by another layer. In the case of smooth surfaces ofsimple shape, the reactive layer can also be applied in a form in whichit is bound to smooth absorbing paper, which at the same time acts as ahigh-contrast background. After the chemical recording of the paperimage, the chemigram," is stripped off. Chemigraphy means the effect ofthe reagent on the reactive layer. In most cases color photography isused for documenting the chemigram.

According to one embodiment of the invention. structural bodies with aplane, developable surface are provided with cemented-on coated metal orplastic foil. in the reactive layers prepared with binder, nosegregation of the indicators occurs. In the case of liquidcrystalreactive layers, this is achieved by spraying them on in a low-boilingsolvent so thinly that the solvent evaporates immediately leaving a dryfilm on the body.v A sprayed-on gelatine sol mixture results in aslightly grained surface with a stipple effect in the flow patternimage. However, the image of the boundarylayer flow pattern proper isnot impaired thereby.

The display of the flow around experimental profiles turns out very wellon glass plates coated with a reactive layer. If a suitable fluoride isused as a buffer with an acid reagent, an etched pattern is produced onthe glass, which becomes visible after the reactive layer is removed.

The chemical reagents of the indicator system are gases or such vaporswhich have a saturation vapor pressure at the operating temperaturewhich is at least sufficient for the indication. In most cases, theoperating temperature is room temperature. The chemical reagents can beadded in gaseous form to the flowing medium from pressure bottles. Inother cases it is sufficient to charge the gas or air stream byconducting it over, or bubbling it through, an aqueous solution ofthereagent.

The generally homogeneous admixture of a chemical reagent can beachieved, for instance, by introducing it ahead of a mixing blower, orby inserting a section of tubing for the mixing.

For liquid-crystal layers, operation in a closed circuit is recommended,in order to keep the concentration of the reagent completely constantthrough regulation. The required partial pressure is several hundredTorr. In addition to depending upon the specific reagent, the magnitudeof the approrpriate partial pressure also depends upon the compositionof the liquid crystal and on how high above room temperature their rangeof thermographic indication is. A separation of about 10C is sufficientfor this purpose.

The observation and recording of liquid-crystal chemigrams isaccomplished during the flow exposure by means of optical devices.

Suitable indicator systems for making the flow visible are, according tothe invention, acid-base indicators, such as phenolphthalein, thymolblue and other indicators or indicator mixtures, which are prepared byknown methods or are commercially available. Furthermore, mixtures suchas those used for pH measurements have been found to be particularlygood, since they have a color spectrum full of nuances. This spectrum istraversed progressively from one extreme color to another during theflow exposure.

It has also been possible to obtain good recordings by the method ofthis invention with a four-component indicator according to Yamada, asdescribed in J. chem. Educat. (1937), p. 275. This mixture produceschemigrams rich in color nuances and therefore furnishes highlydifferentiated flow images. It is therefore suited particularly for flowfields with areas of very different velocities.

The Universal Indicator Merck pH paper has also been found to beparticularly well suited. This mixture is colored yellow-red for acidreagents and is suited particularly for the detection of turbulentboundarylayer flow. In other cases a two-component indicator forultraviolet light was found to be particularly advantageous.

The pH indicators can be used with any direction of change desired.Hydrochloric-acid vapor has been found particularly well suited as anacid reagent, but other strong volatile acids can also be used, if theycan be mixed well into the gas stream.

Carbon dioxide merits special interest as a noncorrosive, easilyhandled, inexpensive and readily available reagent. It can be taken fromsteel bottles or developed from dry ice. Because of the weak acidity,carbon dioxide requires special reactive layers which respondparticularly to undiluted reagents. Of the acid-base indicatorsmentioned, thymol blue is suited particularly. Durable flow recordingscan be obtained with hydrazine. Here, the shift of the Redox potentialis made visible, for instance, by means of crystal purple. Recordingswhich regress after exposure to air, but are reversible, are obtainedwith the acid-base indicators whose transition interval is between a pHof about and 7.2. These are, for example: thymol blue (whose secondtransition is obtained only by strong acids), a cresol red andphenolphthalein with color changes from blue via green to yellow, orred-purpose to yellow, as well as red through pink to colorless. Thesereactive layers record in about 30 seconds. The flow pattern imagedecays completely within about three minutes. With very goodpreparation, the time is just sufficient for photographmg.

As an alkaline reagent, ammonia from a pressure bottle is suitable, oralso a vapor such as that which escapes from its acqueous solution. Itis less corrosive than hydrochloric acid. The volatile methyl amines,mono-, diand tri-methyl amines, can also be used.

The development of the flow pattern takes several seconds to about fiveminutes and therefore proceeds rapidly and unequivocally. The time ofthe color reaction depends on the concentration of the reagent in themedium and, finally, on the state of development desired.

The indicator color for an acid reagent changes are as follows:

YAMADA: Green-yelloworange-red MERCK: Blue green-green-ocher-yellow-redThymol blue: Blue-green-yelIow-red UV light: Green yellow-light blueCrystal purple: Colorless/light blue-blue (C0 The first two mixtures canbe adjusted in themselves to blue by alkali. This color adjustment soonshifts under the influence of the carbon dioxide in the air toward tothe green coloration mentioned. A particularly good record is obtainedof the flow pattern by the addition of alkaline or acid buffersubstances to the indicator mixture. For adjusting the basic color hue,a small amount of alkali or acid is often sufficient. Potassium ortetramethylammonium fluoride also buffers hydrochloric acid. Citric acidor sodium hydrogen sulfate buffers ammonia well.

Redox systems with the reagents ozone, oxygen, chlorine, bromine,nitrogen dioxide can be applied in interaction with reducing agents suchas thiosulfate,

sulfite and, for the reagents sulfur dioxide, hydrazine, hydroxylamine,in interaction with oxidants such as iron (IlI)-, manganese (IV)- orcerium (IV) salts. They must be indicated by means of an organic Redoxindicator, such as ferroin.

Mixtures of cholesterinic liquid crystals are also suitable for flowindication according to the method of the present invention. Ahalogenated methane derivate can be used as the reagent. Suitablesubstances are commercially available pure and as a composition (for agiven temperature range). In particular, chlorine and bromine methane,methylene chloride and chloroform are highly effective reagents. Thesevapors diffuse reversibly into the reactive layer. They change theirphase state in the process and produce a color spectrum from colorlessthrough red, yellow, green, blue, violet to colorless through refractionand circular polarization of the incident light. The boundary-layer flowfield image is produced instantaneously and therefore offers thepossibility to obtain a survey of the dynamic flow behavior as differentboundary conditions are traversed. On the other hand, the observation ofstationary flow conditions is possible only during the exposure.

Depending on the nature of the flow problem to be investigated, one orthe other indicator system is advantageous for making the flow processvisible. If the recording of stationary flow conditions, which ispossible only by photographic means, is very difficult, or if the flowconditions of interest are already known, one would choose an acid-baseindicator system, where one can work with a daylight or ultravioletindicator. If a white contrast can be obtained easily, it is best to usethe daylight acid-base indicators because they furnish an easily visiblechemigram. If white contrast is not pos' sible, dark contrast is chosenand is artifically enhanced, if necessary. The surface of the structuralpart is prepared with the reactive layer of the ultraviolet mixedindicator. If there is high corrosion sensitivity, carbon dioxide ischosen as the reagent.

The method of this invention can serve for rapid and simple checking inthe aerodynamic design of moving and stationary bodies in equipment andmachinery which are exposed to flow or generate flow, such as, vacuumcleaners, heater fans or blowers. One obtains chemical recordingsregarding the patterns of the boundary-layer flow in the equipment beinginvestigated. The engineering design of bodies which are exposed to flowor generate flow is thereby facilitated.

The invention will be explained more fully by means of the followingexamples relating to the preparation of indicator mixtures and the useof such indicator mixtures:

PREPARATION OF INDICATOR MIXTURES Yamada Mixed Indicator 45 g ofgranular gelatine MERCK No. 4078 are dissolved in 450 g of water whilestirring and heating in a water bath, and are mixed with g of glycerinMERCK No. 4091. Then 1.6 g of 4-hydroxy benzoic acid methyl ester MERCKNo. 6757 (preservative) is dissolved in a small amount of ethanol, andthe solution is added to the gelatine so]. In about 200 ml of ethanolone now dissolves mg thymol blue 250 mg methyl red 1200 mg brominethymol blue, and

2000 mg phenophthalein, and the basic color is adjusted for green-blueby means, for example, of a few centiliters of n/ 10 sodium hydroxide.Finally, the dye solution is stirred into the gelatine solution soslowly that no gelatine precipitates in the process.

Universal Indicator as per MERCK No. 9527 8 rolls of original IndicatorPaper are bleached in 200 ml of lukewarm water in a 250-ml beaker. Thedecanted solution is evaporated to the limit of solubility, in order toobtain the highest possible area concentration of the indicator.

45 g of gelatine powder MERCK No. 4078 in 450 ml of the indicatorconcentrate thus obtained, are dissolved. Depending on the choice of thereagent, i.e., whether the development is carried out with hydrochloricacid or ammonia, the basic color of the indicator mixture is adjusted toblue-green with sodium hydroxide or to red with citric acid.

Ultraviolet Mixed Indicator 100 g of gelatine are dissolved in 400 g ofwater and are mixed with 560 g of glycerin. Then, 3 g of chinidiniumchloride SCHUCHARDT No. CH 030 are dissolved in warm water, andseparately, 5 g of dichlorfluorescin MERCK No. 9676, are dissolved inethanol, and each is introduced into the gelatine sol. The mixture isfluorescent in the yellow-green base color and is ready for use with anacid reagent. During the chemical recording, the color turns tolight-blue in ultraviolet light, while it remains unchanged in daylight.

l.iquid-(.rystal Indicators 3.0 g cholestcryl-oleate RIEDEL 3630l 0.8 gcholesteryI-pclargonatc RIEDEL 36302 0.5 g cholcslcryl-henzoatc RIEDEL36304 0.4 g choIcstcryl-chloridc RIEDEL 36305 are dissolved in about I00ml of petroleum benzine, Kp 50 to 75, MERCK 1773. The thermographicchange is at 30 to 32C. The solution is suitable for immersion as wellas for spraying on.

INDICATORS FOR CARBON DIOXIDE Crystal-Purple Indicator 1 g of granulatedgelatine MERCK No. 4078 is dissolved in 10 g of glycol or glycerin atelevated temperature and under prolonged stirring. Then, about 100 mg ofcrystal purple are also dissolved and adjusted, after cooling, to thebasic color, light-blue, by about 0.7 ml of hydrazine hydrate. Theindicator mixture is always prepared fresh and protected from the air.

Methylviolet Indicator This is prepared exactly like the crystal purpleindicator and adjusted to a pale violet.

Thymol Blue Indicator Into the water-free glycol or glycerin-gelatinesolution described previously, 300 mg of thymol blue are Stirred anddissolved. A few drops of concentrated sodium hydroxide are added andthe base color is thereby adjusted to blue.

This mixture can also be used with a strong acid as the reagent and thenindicates with a vivid contrast similar to the Universal Indicator byMerck. If one leaves the yellow basic color of the indicator, themixture is ready for use with a basic reagent. Transition is from greento blue, reversible with ammonia.

Cresol Red Indicator In the glycol or glycerin gelatin previously, about5 mg of cresol red are dissolved. The basic color is adjusted to violetby means of concentrated sodium hydroxide.

Phenolphthalein Indicator The glycol or glycerin gelatin described ismixed with about mg of phenolphthalein and adjusted to the basic colorred by adding about 15 mg of sodium hydroxide, dissolved in a littlewater.

Besides the indicators and indicator mixtures described, otherindicator-dye mixtures can also be used for the method according to theinvention, if they make differences of the flow behavior recognizable ina satisfactory manner.

EXAMPLES OF THE USE OF SUCH INDICATOR MIXTURES Example 1 A glass plate10 cm X 10 cm is Coated with a reactive layer, about 0.1 mm thick, ofthe Yamada mixed indicator described previously. This can be done bypainting it several times with a brush. After a gelling time of about 5minutes, an air stream of about 3 l/min, to which 10 to 30-Torr HCI hasbeen added, is blown over the plate at a glancing angle from apolypropylene nozzle of 2 mm inside diameter. With this reagentconcentration a multi-colored jet cone, as shown in' FIG. I, isgenerated in about 5 sec of exposure.

Example 2 An anodized (eloxadized) A1 sheet of2| cm X 35 cm is coatedwith a reactive layer of about 0.05 mm thickness of the MERCK UniversalIndicator mixture described previously, with the addition of 3,u.moI/cmof tetraethyl ammonium fluoride. Then the coating is dried slowly bycovering up and a prepared surface with only little development of adrying ring at the edges is obtained. The metal sheet was cut intorectan gles of 30 mm X 76 mm and serves for experiments on the influenceof flow velocity and pressure, respectively. From a No. 14 injectionneedle an amount of 100 ml of air, mixed with 5 percent by volume ofhydrochloric acid gas, is blown through a polypropylene nozzle over thelength of the rectangle at a glancing angle. The initial exit velocityis about l6 rn/sec. In about 5 seconds a red jet cone with yellow edges,as shown in FIG. 2, is produced.

Example 3 A wooden board of about 10 cm X 10 cm is painted black and thedried board is coated with the ultraviolet mixed indicator describedpreviously in a thickness of about 0.2 mm. The layer dries in about 5minutes and glows yellow-green under an ultraviolet lamp. From apolypropylene nozzle of 2 mm inside diameter an air stream of about 3l/min, to which about 30-Torr HCI has been added, is blown over theplate. In about 10 seconds, an ultraviolet-blue jet cone as shown inFIG. 3, is generated.

Example 4 A glass plate of 10 cm X 10 cm is coated with an almostcolorless reactive layer, about 0.1 mm thick, of the crystal-purpleindicator described previously. After a gelling time of about 15minutes, a carbon dioxide jet developed from dry ice is blown from the2-mm diameter polypropylene nozzle over the layer at a glancing angle.The flow cone is indicated blue, somewhat more intensely at the core.The methylviolet indicator reacts in the same manner but with a violethue. In air, the flow recording disappears in the increasingly bluebackground, but can be preserved longer without carbon dioxide.

Example The same glass plate as used in Example 4 is coated with a bluereactive layer, about 0.1 mm thick, of the thymol blue indicatordescribed previously. After a gelling time of about minutes, a carbondioxide jet developed from dry ice is blown from the 2-mm diameterpolypropylene nozzle over the layer at a glancing angle. In about l5seconds, the image of a yellow jet cone of, for instance, 3 cm lengthappears. At the same flow rate, the gas escapes from a No. 14 tubularneedle several times faster and is visible as a yello-green cone, about7 cm long, with a yellow core. The recordings disappear in air in about2 minutes, as the base color reappears.

An air stream mixed with HCl is recorded in several colors with a redcore and is permanent. If a reactive layer which has been left yellow isapplied, then an air jet mixed with ammonia is recorded as green-blue.This coloring is reversible, like the one with carbon dioxide, anddisappears in air.

Example 6 A glass plate like the one in Example 4 is coated with aviolet reactive layer about 0.1 mm thick of the cresolred indicatordescribed previously. After a gelling time of about 15 minutes, a carbondioxide jet is blown over the layer from a tubular needle at a glancingangle. The jet cone is recorded immediately with a yellow color. Theboundary layer image disappears in about 1 minute after exposure andreappears after repeated action.

An air stream mixed with I-ICl is permanently recorded green and yellowwith a red core.

Example 7 A glass plate as in Example 4 is coated with a red reactivelayer about 0. I mm thick of the phenolphthalein indicator describedabove. After a gelling time of about 15 minutes a carbon dioxide jet isblown from a tubular needle over the layer at a glancing angle. In aboutone minute a pinkjet cone with a colorless core is recorded. The imagedisappears reversibly in about 2 minutes.

Example 8 A plate of black polyethylene 10 cm X cm is made insensitiveagainst diffusion shifts of the thermographic transition interval byspraying on a black primer RIE- DEL DE HAEN No. 36300, diluted I 5 withwater. The spraying on ofa liquid-crystal reactive layer about 12 ,umthick is controlled for high color brilliance through brief heating bymeans of a hot-air blower. Then, an air stream of about 3 l/min, towhich ZOO-Torr chloroform vapor has been added, is blown over the plateat a glancing angle, as was described in Example l. With goodillumination, a boundary layer flow pattern image appears which isirridescent in the colors blue-green-red and the fluctuations of whichmirror as to direction, intensity and color all changes of the air jet.A sketch showing this is reproduced in FIG. 4.

Example 9 The rotor of a RAPID vacuum cleaner consists of a circular,l3-cm base plate and a cover plate with an intake opening of 5 cmdiameter. Strips of aluminum sheet are riveted centrically about thecenter between the two plates as the blades and throw off the airradially. A vinylan foil is cemented to the base plate and is coated bymeans of an impregnated circular filter with Universal Indicator byMERCK. After reassembly and a drying time of 5 minutes, the full powerat 220 V is switched on and the entering air stream is conducted over aglass dish filled with 32 7: hydrochloric acid. where it is charged with5 to 10 Torr of HCI. In about 30 seconds the chemically recorded image,shown in FIG. 5, of the air is produced which hits the base plate. flowspreferably along the concave blades and is recorded in yellow green-redcolors.

Example 10 The same rotors as described in Example 9 are dyed on thenatural aluminum oxide skin with the MERCK Universal Indicator describedpreviously. The exposure with 32-% hydrochloric acid is extended toabout 5 minutes, until an image similar to that shown in FIG. 5 becomesvisible.

Example II The horizontal and vertical, plane and curved interiorsurfaces of an automobile fan housing molded of black plastic and thefive highly curved blades of its rotor are investigated with the MERCKindicator mixture described previously. For the propeller, the MERCKindicator gel and the fluorescent indicator gel proved useful. The bladesurfaces very visibly change color from yellow to red and from green toblue, respectively, as shown in FIGS. 6 and 7.

Example 12 The housing of a RAPID kitchen beater is made oflight-colored plastic and has only curved surfaces. The interiorsurfaces are prepared by painting or spraying with MERCK and YAMADAmixture described above. By chemical interval recording (2 to 3 secondsper step) a series of images are observed and photographed in color.Both reactive layers clearly indicate chemigraphically the main flowpattern. The propeller and two gears near the wall show zones ofturbulence. The partition situated in front of the handle prevents flowthrough the handle and forces the cooling air to flow around to motor.Although this main flow is disturbed by the electric cable, it takes apath about the motor shaft. Part of the air also leaves at the openingfor the stirrer chuck, while the main quantity escapes through theopening provided, which can be documented very well in the color photo.

Example l3 Three blower rotors with three curved, saddleshaped blades ofblack plastic, with six similar blades and with ten straight blades ofblack-enamelled metal were coated with the mixed indicator forultraviolet light described previously, for the purpose of recording theboundary layer flow. They were exposed, always at full motor power (220V), to an air stream mixed with about ZO-Torr HCl, and subsequently showin the dark, as observed under ultraviolet light (DESAGA lamp), colorchanges toward blue at various surface areas of the blades. The UV lightchemigram could also be recorded by color photography with an exposureof several minutes.

Example 14 A wing profile and a cylinder, as known models in flowdynamics, were investigated in their projections on the plane. To thisend the profile bodies were mixed vertically on a glass plate coveredwith a daylight indicator and the plate was suspended from the top in astanding open Plexiglass tube serving as a wind tunnel.

The blower is at the lower end of the tube. After switching the bloweron, the exposure is begun by pushing a glass dish filled with 30 ml ofreagent solution (36-% hydrochloric acid) under the intake opening ofthe tube. The boundary layer flow pattern is chemically recorded whilebeing monitored visually. A sharply outlined flow pattern of the wingprofile is obtained. A picture prepared with the Yamada mixed indicatoris shown in FIG. 8. FIGS. 9 and 10 show pictures which were obtainedwith a flow-exposed cylinder at laminar flow of different quality withpaper impregnated with MERC K indicator. In this manner the effect of arectification grid can also be made clearly visible by means of chemicalrecording with the MERCK Universal Indicator. At first, the flow in FIG.9 appears superimposed by a rotary motion and residual turbulence, whilein FIG. 1 0 it is largely free of this as a result of the installationof the grid.

Example IS The nature of the laminar flow in a l-m long wind tunnelconsisting of a Plexiglass tube of 14 cm diameter was investigated. Thisis accomplished by cementing-in a filter paper strip 15 to cm wide whichwas impregnated with the MERCK indicator mixture described previously.The chemical recording takes place with hydrochloric vapor as describedpreviously. The zone behind the blower is reddened most intensely in anonuniform, cloudy manner. The cloud effect diminishes only slightlytoward the exit. Intense reddening starts out from one edge of the paperand this indicates rot'ation of the streaming air. After installation ofa rectification grid, consisting of Plexiglass tube sections of 16 mmdiameter and length with rounded ends, the reddening is shortened to ashort section (about 20 cm) behind the rectifier. The main length iscolored almost uniformly orange-yellow and therefore indicates laminarflow. The settling of the air stream can further be seen from a glassplate coated with a reactive layer, which is suspended in the bloweropening. Its front and side edges are now colored much more uniformly,but not yet symmetrically, as the chemical recording shows clearly. Adistinct asymmetry also remains at the cylinder profile. By extendingthe wind tunnel to 2 m, the rotation is eliminated and the air flowsaround the cylinder ina very good laminar flow.

....- is aim dm s- 1. A method for making boundary-layer flow conditionsof gases visible, comprising the steps of:

a. applying at least one chemical color indicator in a thin reactivelayer to the surface of a structural body to be exposed to the gas flow;and b. then subjecting the structural body to the gas flow for thepurpose of producing a color reaction of the color indicator to the gasand thereby a visible boundary-layer flow image. I 2. The methodaccording to claim I, and further comprising the additional step ofrecording the bounda ry-layer flow image after it has been made visible.7

3. The method according to claim 1, and further comprising theadditional step of photographing the boundary-flow image in color afterit has been made visible.

4. The method according to claim 1 wherein the chemical color indicatoris applied by binding it to a smooth absorbent paper and then applyingthe paper to the structural body.

'- 5 The method according to claim 4 and further comprising theadditional step of stripping said paper off the structural body afterthe structural body has been subjected to the gas flow and after achemical color image of the boundary-layer flow has been produced on thepaper. thereby providing a record of the flow on the paper.

6. The method according to claim I wherein the chemical color indicatorapplied is an acid-base indicator.

7. The method according to claim 6 and further comprising the additionalsteps of mixing a buffer with the acid-base indicator prior to applyingthe acid-base indicator to the structural body.

8. The method according to claim 6 and further comprising the additionalstep of mixing an alkaline buffer with the acid-base indicator prior toapplying the acidbase indicator to the structural body.

9. The method according to claim 6 and further comprising the additionalstep of mixing an acid buffer with the acid-base indicator prior toapplying the acid-base indicator to the structural body.

10. The method according to claim 1 wherein the chemical color indicatorapplied is a Redox indicator. llfThe method according to claim 1 whereinthe chemical color indicator applied is a liquid-crystal indicator.

12. The method according to claim 11 wherein the liquid-crystalindicator is applied by spraying it on thinly in a low-boiling solventwhereby the solvent evaporates, leaving a dry film on the structuralbody.

13. The method according to claim 11 wherein the liquid-crystalindicator applied is a cholesterinic liquid.

14. The method according to claim 1 and further comprising theadditional step of adding a reagent to the gas flow prior to subjectingthe structural body to the gas flow. V

15. The method according to claim I4 wherein the reagent is added ingaseous form to the gas flow.

16. The method according to claim 15 wherein the reagent is added to thegas flow by conducting the gas flow over the gaseous reagent.

17. The method according to claim 14 wherein the reagent is added to thegas flow by bubbling the gas flow through an aqueous solution of thereagent.

18. The method according to claim 1 wherein the chemical color indicatorapplied is applied in a moisture-retaining additive.

19. The method according to claim 1 wherein the chemical color indicatoris applied in combination with a gelling agent and a moisture retainer.

20. The method according to claim I and further comprising theadditional step of applying a foil to the structural body prior tosubjecting the body to the gas flow.

21. The method according to claim 20 and further comprising theadditional step of coating the foil with a binding agent prior toapplying the foil to the body.

22. The method according to claim 1 and further comprising theadditional step of applying a metal foil to the structural body prior tosubjecting the body to the gas flow.

23. The method according to claim I and further comprising theadditional step of applying a plastic foil to the structural body priorto subjecting the body vto the gas flow.

t in o-mso (10-69) fAttesti-ng Officer".-

UNITED STATES PA ENT OFFICE I CERTIFICATE OF CORRECTION Patent No.13,787,874 I I I Dated 9? Inventor(s) rd Urban 7 It is certified" thaterror appears in the above-identified patent and that saidLette s Patentare hereby corrected as shown below:

In the Foreign Application Priority Data. change the file number ofthe-German application upon which the claim for priority is based-from'German ........2133865" to I a o I. 0 0 0 1 v I I column 1111565,-change mix ed to --fixec1 In column 12, line 1 i 1a1m'7)., change'f"a.ddit a ep ..-addit iona1 step v v I p j I I p Signed aha sealedthis 9th day of July "197 (SEAL).

v Attest:

' c. "MARSHALL MN MC Y Mi. GI N; e i Commissioner of Patents uscoMM-ocwen-Poo .15. G'VIIIIIIN PRINTING .IIICI I... O-Sll-SSL UNITED STATESPATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,787,874 Dated1/22/197 Inventor(s) Gerd Urban It is certifiedthat error appears in theaboveidentified patent and that said Letters Patent are hereby correctedas shown below:

In the Foreign Application Priority Data. change the file number ofthe'German' application upon which the claim for priority is based-from"German ........2l33865" to cane 00000 1 In column 10, line 65, change"mixed" to "fixed-- In column 12, line '14 (claim 7)., change':"additional steps? "to --a.dditiona1 step i- Signed and sealed this9th day of July 1974.

y (SEAL) Attest: v McCOY MI. G Bs0N,"J-R,.i' .C. :'MARSHALL DANN I tingOffiQI-IIv v I "Commissioner' ofPatents ORM Po-1 5 10-6 0 0( uscoMM-Dcoosnwoo v u.s. covllmlnn manna omen I... o-su-su.

2. The method according to claim 1, and further comprising theadditional step of recording the boundary-layer flow image after it hasbeen made visible.
 3. The method according to claim 1, and furthercomprising the additional step of photographing the boundary-flow imagein color after it has been made visible.
 4. The method according toclaim 1 wherein the chemical color indicator is applied by binding it toa smooth absorbent paper and then applying the paper to the structuralbody.
 5. The method according to claim 4 and further comprising theadditional step of stripping said paper off the structural body afterthe structural body has been subjected to the gas flow and after achemical color image of the boundary-layer flow has been produced on thepaper, thereby providing a record of the flow on the paper.
 6. Themethod according to claim 1 wherein the chemical color indicator appliedis an acid-base indicator.
 7. The method according to claim 6 andfurther comprising the additional steps of mixing a buffer with theacid-base indicator prior to applying the acid-base indicator to thestructural body.
 8. The method according to claim 6 and furthercomprising the additional step of mixing an alkaline buffer with theacid-base indicator prior to applying the acid-base indicator to thestructural body.
 9. The method according to claim 6 and furthercomprising the additional step of mixing an acid buffer with theacid-base indicator prior to applying the acid-base indicator to thestructural body.
 10. The method according to claim 1 wherein thechemical color indicator applied is a Redox indicator.
 11. The methodaccording to claim 1 wherein the chemical color indicator applied is aliquid-crystal indicator.
 12. The method according to claim 11 whereinthe liquid-crystal indicator is applied by spraying it on thinly in alow-boiling solvent whereby the solvent evaporates, leaving a dry filmon the structural body.
 13. The method according to claim 11 wherein theliquid-crystal indicator applIed is a cholesterinic liquid.
 14. Themethod according to claim 1 and further comprising the additional stepof adding a reagent to the gas flow prior to subjecting the structuralbody to the gas flow.
 15. The method according to claim 14 wherein thereagent is added in gaseous form to the gas flow.
 16. The methodaccording to claim 15 wherein the reagent is added to the gas flow byconducting the gas flow over the gaseous reagent.
 17. The methodaccording to claim 14 wherein the reagent is added to the gas flow bybubbling the gas flow through an aqueous solution of the reagent. 18.The method according to claim 1 wherein the chemical color indicatorapplied is applied in a moisture-retaining additive.
 19. The methodaccording to claim 1 wherein the chemical color indicator is applied incombination with a gelling agent and a moisture retainer.
 20. The methodaccording to claim 1 and further comprising the additional step ofapplying a foil to the structural body prior to subjecting the body tothe gas flow.
 21. The method according to claim 20 and furthercomprising the additional step of coating the foil with a binding agentprior to applying the foil to the body.
 22. The method according toclaim 1 and further comprising the additional step of applying a metalfoil to the structural body prior to subjecting the body to the gasflow.
 23. The method according to claim 1 and further comprising theadditional step of applying a plastic foil to the structural body priorto subjecting the body to the gas flow.